Extensive developments have occurred in imaging elements based upon non-silver, energy-activatible materials that generate amines to react to form a dye. Examples of such elements are set forth in Research Disclosure, Vol. 184, Pub. No. 18436, Aug., 1979, published by Industrial Opportunities Ltd, Homewell, Havant, Hampshire, United Kingdom. Particularly preferred are those in which o-phthalaldehyde, hereinafter phthalaldehyde, is included in the imaging composition to react with the imagewise-released amines to form the dye.
Although such imaging elements have proven to be highly successful, there have been aspects of these elements for which improvement has been sought. A particular problem has been the volatility of the phthalaldehyde. Such volatility has required some selectivity in the binder used with the composition. U.S. Pat. No. 4,247,625, issued on Jan. 27, 1981, describes a polysulfonamide binder that has proven unexpectedly superior retention of the volatile phthalaldehyde. Overcoats that aid further in retaining the phthalaldehyde have also been described. Nevertheless, such specialized binders and overcoats have added to the cost of the element.
Yet another aspect of these imaging elements has been the dimensional changes that occur when the element is heated to cause dye development, for example, at temperatures exceeding 100.degree. C. Some of these changes can be shown to be image dependent--that is, that greater changes occur in the areas of D.sub.max than in the areas of D.sub.min. Such dimensional changes are troublesome if multiple image registration is desirable, such as in the use of separation negatives or positives to create a composite image.
What has been desired, then, prior to this invention, is an imaging element that relies upon the reaction of phthalaldehyde with an amine to form dye, without using the volatile form of phthalaldehyde and without creating D.sub.max areas having a dimensional response to thermal processing that is different from that of the D.sub.min areas.
There have been disclosed in the literature certain adducts of phthalaldehyde that are not as volatile as phthalaldehyde itself. J. Org. Chem., Vols. 30, 35 and 43, p. 2251 et seq, p. 3940 et seq and p. 3838 et seq.; (1965), (1970) and (1978), respectively, disclose, among others, 1-hydroxy-3-benzamidophthalan, 1-hydroxy-3-(4-chlorobenzamido)phthalan, and N-acetyl-1,3-dihydroxyisoindoline. However, these adducts are described as releasing phthalaldehyde only at very high temperatures, or as having a very complex decomposition reaction. Therefore, prior to this invention, there was reason to believe such adducts could not be relied upon to release the phthalaldehyde necessary to combine with an amine to form a dye at lower temperatures, e.g., 135.degree. C. Furthermore, it has been discovered that, although some of these prior art compounds can in fact act as a source of phthalaldehyde, these prior art compounds produce a maximum neutral density that is less than the amount needed for preferred uses.